The Impact of the New IAU Resolutions on ICRF Definition and Realization

Following the adoption of the International Celestial Reference System and Frame (ICRS and ICRF) by the IAU in 1997, several resolutions on reference systems have been passed by the IAU in 2000 and 2006 and endorsed by the IUGG in 2003 and 2007, respectively. These resolutions concern especially the transformation between the International Terrestrial Reference System (ITRS) and the Geocentric Celestial Reference System (GCRS) that is essential for realizing the ICRS from directions of extragalactic radio sources observed from the Earth by VLBI. First, the IAU 2000 resolutions have refined the concepts and definition of the astronomical reference systems and parameters for Earth's rotation, and adopted the IAU 2000 precession-nutation. Then, the IAU 2006 resolutions have adopted a new precession model that is consistent with dynamical theories and have addressed definition, terminology or orientation issues relative to reference systems and time scales that needed to be specified after the adoption of the IAU 2000 resolutions. These in particular provide a refined definition of the pole and the origin on the equator as well as a rigorous definition of sidereal rotation of the Earth. These also allow an accurate realization of the celestial intermediate system that replaces the classical celestial system based on the true equator and equinox of date. There was an additional IUGG 2007 resolution for the terrestrial reference system. Finally, the IAU 2009. Transactions of the IAU XXVIIB. Rio de Janeiro, Corbett I (ed) vol 6. Cambridge University Press, pp 55-70) resolutions have adopted a new system of astronomical constants - including conventional values of the IAU 2000/2006 resolutions - and adopted the Second Realization, ICRF2, of the International Celestial Reference Frame. This paper recalls the main aspects of these recent IAU resolutions as well as their consequences on the concepts, definitions, nomenclature and models that are suitable for modern realizations of reference systems. The impact of these resolutions on the definition and realization of the International Celestial Reference Frame (ICRF) is described.